Damped rotary encoder

ABSTRACT

A tuned damped absorber arrangement is used to attenuate the response of an encoder. A vibration problem occurs on rotary axis heads which are used in machining operations. When using long slender tools, a vibration can be created which causes the encoder to lose its position. The device uses a tuned damped absorber that is attached to the input shaft of the encoder to dampen unwanted vibration.

FIELD

A damper device is used to prevent a rotary encoder from losing itsposition as a result of resonant vibrations.

BACKGROUND

A tuned damped absorber arrangement is used to attenuate the response ofan encoder. If a vibration disturbance is of a high amplitude and/or ifprocessing conditions result in a resonant condition, the encoder canlose its position. Excessive vibration can also damage the internalelectronic components of the encoder.

The vibration problem occurs on rotary axis heads which are used inmachining operations. When using long slender tools, a vibration can becreated which causes the encoder to lose its position.

Encoders by their nature should not be isolated from the axis which theyare controlling since to do so would potentially result in a loss ofposition indicating accuracy. The device relies on the use of a tuneddamped absorber in a unique configuration for its operation. The deviceexhibits a significant attenuation of unwanted vibrations that are bothlateral and torsional. The improved response eliminates the lost axisposition situation and improves the reliability and life of the encoder.

In one application, the mounting of the encoder on a fairly slender beamexacerbates the problem of resonant vibration. The damper deviceaddresses the vibration exhibited at the encoder under both anunbalanced response and during cutting operations. One embodimentaddresses torsional response, another addresses the lateral response ofthe beam to which the rotating part of the encoder is mounted.

It is believed that tool vibration is exciting the encoder duringchatter and causing mechanical noise on the encoder output signals. Ifthe vibration that normally affects the encoder can be attenuated by adamper without affecting other aspects of velocity loop bandwidth, thedamper should eliminate the problem.

The vibration problem has appeared typically in cutting applications ofaluminum at a fairly high speed and is often more prevalent when the cutresults in tool chatter. In one application, a long slender tool is usedwith resulting chatter that is approximately equal to the torsional modeof the input shaft of the encoder. The chatter has been known to causethe rotary axis of the encoder to drift, thus giving an incorrectreading.

When chatter occurs, the A-axis position of the encoder drifts.Vibration of the input shaft of the encoder is the likely root cause ofthe drift. It is believed that the chatter frequency is in the 1000 to2000 Hz region for tool modes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows an end view of an absorber for a rotary encoder that dampstorsional vibrations.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a side view of an absorber for a rotary encoder that dampslateral vibrations.

DETAILED DESCRIPTION OF THE DEVICE

FIGS. 1 and 2 show an end view and a sectional view, respectively, of anabsorber 12 for a rotary encoder that damps torsional vibrations. Amounting plate 14 is mounted on the end of the scale post or input shaft16 of a rotary encoder by a plurality of bolts 17. The input shaft 16has a central axis 15. A torsional mass plate 18 is coupled to themounting plate 14 by a plurality of shoulder bolts 20, and issymmetrically positioned relative to the axis 15 of the shaft. Eachshoulder bolt 20 passes through a mounting aperture 22 formed in amounting flange 23. The mounting aperture 22 has a larger diameter thanthe diameter of the body 21 of the shoulder bolt 20 that passes throughit.

As best seen in FIG. 2, a washer 25 is used on the end of each of theshoulder bolts 20, and the washer presses against a first outerresilient donut 26 that presses against the outer side 27 of themounting flange 23. A second inner resilient donut 28 is capturedbetween the inner side 29 of the mounting flange 23 and the mountingplate 14. The diameter of the center of the hole 30 in the center of thedonuts 26 and 28 is larger than the diameter of the body 21 of theshoulder bolt so that the body of the shoulder bolt does not touch thebody of either of the donuts 26 and 28. An outer spring washer 31 ispositioned between the head of the shoulder bolt 20 and the washer 25,and an inner spring washer 32 is positioned between the body 21 of theshoulder bolt and the mounting plate 14. The inner and outer donuts 26and 28 isolate the torsional mass plate 18 from the mounting plate 14and the input shaft 16, and torsional mass plate dampens torsionalvibration which may exist in the input shaft.

FIG. 3 is a side view of an absorber 40 for a rotary encoder that dampslateral vibrations. The absorber 40 mounted on the end of the inputshaft 42 of a rotary encoder having a center axis 43. The absorber 40comprises a ring shaped mass element 44 which is attached to the end ofthe input shaft 42 by a bolt 46 and a retaining washer 48. The masselement 44 is held between an outer resilient donut 50 and an innerresilient donut 52. The inner resilient donut 52 is mounted against anencoder retaining nut or mounting flange 54 that is located on the outersurface of the input shaft 42. The outer resilient donut 50 is mountedagainst the inner surface of the retaining washer 48. The end 47 of thebolt 46 is screwed into the end of the input shaft 42. The outerdiameter of the cylindrical surface 49 of the retaining washer 48 and ofthe input shaft 42 is less than the diameters of the central apertures51 and 53 formed in the donuts and of the mounting aperture 45 in themass element 44 so that the mass element is free to vibrate in responseto vibrations of the input shaft 42. Since the damping mass 44 is asymmetrical ring that is mounted along the axis 43 of the input shaft42, it is able to attenuate both horizontal and vertical vibrations thatmay exist in the input shaft.

Through the use of the damper devices described above, thesusceptibility of the A-axis scale of a rotary encoder to inducedtorsional and lateral vibrations is significantly reduced. Also, becausetorsional and lateral vibrations are significantly reduced, excessivevelocity alarms are completely eliminated or greatly reduced duringencoder operation.

1. A tuned damper for a input shaft on a rotary encoder, the dampercomprising: a mass element coupled to the end of the input shaft themass element being symmetrically positioned around the axis of the inputshaft.
 2. The tuned damper of claim 1 further comprising: a first ringof resilient damping material positioned between the mass material andthe input shaft.
 3. The tuned damper of claim 2 further comprising: afastening element securing the mass element to the input shaft; and, asecond ring of resilient damping material positioned between the end ofthe fastening element and the mass element, wherein the tuned damper iseffective for damping vibrations in the input shaft.
 4. A tuned damperfor the input shaft on a rotary encoder, the damper comprising: amounting plate coupling the damper to the input shaft; a mass platecoupled to the mounting plate; a plurality of fastening elementscoupling the mass plate to the mounting plate; and, first resilientelements separating the fastening elements from direct contact with themass plate, whereby the mounting of the mass plate on the input shaft ofthe encoder damps torsional vibrations in the input shaft.
 5. The tuneddamper of claim 4 further comprising: second resilient elementsseparating the mass plate from the mounting plate, whereby the fasteningelements pass through the first and second resilient elements.
 6. Thetuned damper of claim 5 further comprising: a plurality of mountingflanges formed on the mass plate; and, a mounting aperture formed ineach mounting flange, the first and second resilient elements being incontact with the mounting flanges.
 7. The tuned damper of claim 6further comprising: a plurality of bolts comprising the fasteningelements; and, resilient donuts comprising the resilient elements,wherein the bolts pass through the mounting apertures and the resilientdonuts, and whereby the diameter of the bolts is smaller than thediameter of the mounting apertures and the inner diameter of the donuts.8. The tuned damper of claim 7 further comprising: three boltscomprising the plurality of fastening elements, the three bolts beingspaced equally from one another around the axis of the input shaft. 9.The tuned damper of claim 6 further comprising: a washer positionedunder the head of each bolt, whereby each bolt presses on a washer tocompress the first resilient elements between the washer and mountingflange of the mass plate.
 10. A tuned damper for the input shaft on arotary encoder, the damper comprising: a mounting flange positioned onthe input shaft; a mass element mounted on the input shaft, the masselement being symmetrical and being symmetrically mounted with respectto the axis of the input shaft; and, at least one resilient membermounting the mass element on the input shaft, whereby the tuned damperis able to attenuate horizontal and vertical vibrations that may existin the input shaft.
 11. The tuned damper of claim further comprising: aretaining washer mounted on the end of the input shaft; and a resilientmember between the retaining washer and the mass element, whereby themass element is held on the input shaft between two resilient elements.12. The tuned damper of claim 11 further comprising: a mounting apertureformed in the mass element, the mounting aperture having a diameter thatis greater than the diameter of the input shaft, whereby the resilientmembers position the mass element on the input shaft of the rotaryencoder so that the mass element does not contact the input shaft. 13.The tuned damper of claim 12 further comprising: central apertures inthe resilient members, the central apertures having a diameter that isgreater than the diameter of the input shaft of the rotary encoder,whereby the resilient members do not contact the input shaft.